During NDT/NDI testing, sound traveling through metal from an ultrasound (UT) probe goes through the process of attenuation, absorption and scattering. Flaws of identical size at different depths result in different amplitudes of sound returning to the probe. This is detrimental to quality of the UT inspection and is typically corrected either by a distance amplitude curve (DAC) or a time corrected gain (TCG). In its simplest form, a DAC is a drawing on the screen of an ultrasound instrument that accounts for the loss of sound over the sound path, which is also called the metal path because it is the path sound takes through the part being inspected, usually made of metal. DAC is a curve literally drawn on the screen either manually or digitally by the ultrasonic instrument being utilized by the NDT inspector. TCG is a correction for the DAC curve that makes the curve a straight line by automatically adjusting the gain for flaws of equal size at different sound paths (metal depths) in order that they appear visually at the same screen level on the ultrasonic inspector's screen.
For conventional UT inspection, only the TCG is required, but for Phased Array Ultrasound (PAUT), where beams have variable refraction angles (and hence different transmission coefficients), there is a need for an additional gain correction referred to as the Angle Corrected Gain (ACG). Using a combination of TCG and ACG is a common practice for PAUT inspection because it would be impossible to set every angle and sound path for a multi-transducer probe to the same sensitivity level using TCG only.
The requirement to compensate the gain for both the angle and the sound path length increases the complexity of the PAUT system calibration, which then becomes a time consuming task in the inspection process, requiring skilled and careful technicians. For example, a typical calibration sequence may involve careful scanning of several side drilled hole (SDH) defects over the full angular range at different depths in a reference calibration standard. It must be understood that PAUT is very sensitive to the coupling condition and to the probe orientation on the sample, so each of these scans involves a careful maximization of the signal at each PAUT refraction angle and depth to be used for the inspection. Another aspect to consider is the large differences in gain level required over the angular range, which typically makes it impossible to conduct the calibration scan for one depth with a single gain configuration.
A need therefore exists for a method to reduce the complexity of such a calibration for an NDI Technician in order to have a simpler PAUT calibration. More specifically, the objectives of an improvement to the PAUT calibration would be:                a. To reduce the number of acquisitions required to conduct a PAUT calibration;        b. To reduce the impact of measurement errors in the PAUT calibration process;        c. To improve the precision of the resulting gain compensation;        d. To eliminate the need for manual gain tuning to scan a defect over the full angular range of the PAUT inspection.        